This invention relates to the field of transporting spent nuclear fuel and specifically to maximizing radiation shielding during spent nuclear fuel transfer procedures.
In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. In the commercial nuclear industry, fuel assemblies are typically an assemblage of long, hollow, zircaloy tubes filled with enriched uranium. Upon depletion and subsequent removal, spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, spent nuclear fuel emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times.
Upon defueling a nuclear reactor, spent nuclear fuel is placed in a canister that is submerged in a storage pool. The storage pool facilitates cooling of the spent nuclear fuel and provides radiation shielding that helps contain the emitted neutrons and gamma photons. Generally, canisters are cylindrical steel containers with flat bottoms. A typical canister can hold approximately 24 PWR fuel assemblies or 60 BWR fuel assemblies. When fully loaded with spent nulclear fuel, a canister weighs approximately 45 tons. However, a canister alone does not provide adequate containment of the neutrons and gamma photons emitted by the spent nuclear fuel contained therein. As Such, a loaded canister cannot be further transported from the storage pool without some additional radiation shielding. Because it is preferable to store spent nuclear fuel in a xe2x80x9cdry state,xe2x80x9d the canister must eventually be removed from the storage pool. As such, apparatus that provide additional radiation shielding during transport and long-term dry storage of the spent nuclear fuel are necessary.
In state of the art facilities, additional radiation shielding is achieved by placing the loaded canisters in large cylindrical containers called casks. There are two types of casks used in the industry today, storage casks and transfer casks. A transfer cask is used to transport canisters of spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel in the xe2x80x9cdry statexe2x80x9d for long periods of time. Both transfer casks and storage casks are designed to shield the environment from the neutron and gamma radiation emitted by the spent nuclear fuel through the use of two principles.
First, the gamma radiation emitted by spent nuclear fuel is blocked by placing mass in its way, the greater the density and thickness of the blocking mass, the more effective the attenuation of the gamma radiation. Examples of effective gamma absorbing materials are concrete, lead, and steel. Second, the neutrons emitted by spent nuclear fuel are blocked by placing a material containing hydrogen atoms in their path. As such, any material rich in hydrogen is an effective neutron shield. One example of an effective neutron absorbing material is water.
Guided by the above principles, storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel, size and weight are often secondary considerations (if considered at all). As a result of maximizing the thickness of the gamma and neutron absorbing materials, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh approximately 150 tons and have a height greater than 15 ft. A common problem is that storage casks are often too heavy for the capacity of most nuclear power plant cranes and as such cannot be lifted. Another common problem is that storage casks are too large to be placed in storage pools. Thus, in order to store spent nuclear fuel in a storage cask, a loaded canister must be removed from the storage pool, prepared in a staging area, and transported to the storage cask. Additional radiation shielding is needed throughout all stages of this procedure.
Removal from the storage pool and transport of the loaded canister to the storage cask is facilitated by a transfer cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into an open transfer cask. The canister and transfer cask are then submerged in the storage pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor and lowered into the storage pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The enclosed water provides neutron radiation shielding for the spent nuclear fuel once the transfer cask is removed from the pool. The canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the spent nuclear fuel for storage in the xe2x80x9cdry state.xe2x80x9d Once in the staging area, the water contained in the canister is pumped out of the canister. This is called dewatering. Once dewatered, the spent nuclear fuel is allowed to dry. Once dry, the canister is back-filled with an inert gas such as helium. The canister is then sealed and the canister and the transfer cask are once again lifted by the plant""s crane and transported to the storage cask. The transfer cask is placed atop the storage cask and the canister is lowered through a bottom opening in the transfer cask into the storage cask.
Because a transfer cask must be lifted and handled by a plant""s crane (or other equipment), transfer casks are designed to be a smaller and lighter than storage casks. A transfer cask must be small enough to fit in a storage pool and light enough so that, when it is loaded with a canister of spent nuclear fuel, its weight does not exceed the crane""s rated weight limit. Additionally, a transfer cask must still perform the important function of providing adequate radiation shielding for both the neutron and gamma radiation emitted by the enclosed spent nuclear fuel. As such, transfer casks are made of a gamma absorbing material such as lead and contain a neutron absorbing material. While the pool water sealed in the canister does provide some neutron shielding, this water is eventually drained at the staging area. As such, many transfer casks have either a separate layer of neutron absorbing material or have an annulus filled with water that surrounds the cavity of the transfer cask in which the loaded canister is located.
As stated earlier, the greater the thickness and density of the neutron and gamma absorbing materials, the greater the radiation shielding provided thereby. However, increasing the density and/or thickness of the materials used to make the transfer cask also results in the weight of the cask being increased. Thus, the extent of radiation shielding provided by a transfer cask is directly related to the transfer cask""s weight. The greater the radiation shielding the greater the weight of the cask.
However, the allowable weight of a transfer cask is limited by the lifting capacity of the plant""s crane (or other lifting equipment). The load handled by the crane includes not only the weight of the transfer cask itself, but also the weight of the transfer cask""s payload (i.e., the canister and its contents). A transfer cask must be designed so that the total load handled by the crane during all handling evolutions does not exceed the crane""s rated weight limit, which is typically in the range of 100-125 tons. As such, the permissible weight of a transfer cask is equal to the rated capacity of the plant crane less the weight of its payload. Moreover, it is important to note that when the combined weight of a transfer cask and its payload is equal to the rated lifting capacity of the plant crane, the possible radiation shielding that can be provided by a transfer cask is at a maximum for that particular payload. This is because the thickness of the gamma and neutron absorbing materials are at a maximum for that crane and that payload.
Because the weight of the transfer cask""s payload varies during the different stages of the transport procedure, the permissible weight of the transfer cask is equal to the rated capacity of the plant crane less the weight of the transfer cask""s maximum payload at any lifting step. The weight of the transfer cask""s payload is at a maximum when the transfer cask and canister are lifted out of the storage pool, at which time the canister is full of spent nuclear fuel and water. Thus, according to prior art methods, it is at this stage that the permissible weight of a transfer cask is calculated. The transfer cask is then constructed using this permissible weight as a design limitation.
However, when the transfer cask is set down in the staging area, the pool water is removed from the canister. Upon completion of dewatering the canister, the weight of the transfer cask""s payload is reduced below the rated capacity of the crane, and remains so throughout the rest of the transport procedure. As such, the radiation shielding capacity provided by the transfer cask is sub-par throughout the rest of the procedure when compared to a heavier transfer cask, the weight of which would subsume the available crane capacity. However, a heavier transfer cask can not be used throughout the entire transport procedure because of the fact that the combined weight of the heavier transfer cask and its payload would exceed the rated lifting capacity of the crane during the step of initially lifting the transfer cask from the storage pool. Thus, the maximum amount of radiation shielding is not provided throughout every step of the transfer and dry storage procedure.
While it is possible to transfer the canister of spent nuclear fuel to a heavier transfer cask once the payload is lightened from dewatering, this would take added time, money, effort, space, and equipment. An additional transfer would also increase the amount of radiation exposure to personnel and the chances of a handling mishap. Thus a need exists for a transfer cask that can provide the maximum amount of radiation shielding during all stages of transferring spent nuclear fuel from a storage pool to a storage cask for long-term dry storage, even when the weight of the transfer cask""s payload is varied. A need also exists for a method of transferring a canister of spent nuclear fuel from a storage pool to a storage cask for long-term dry storage that provides the maximum amount of radiation shielding during all stages of the transfer procedure, even when the weight of the transfer cask""s payload is varied.
It is an object of the present invention to provide an apparatus that can provide the maximum amount of radiation shielding during all stages of a spent nuclear fuel transfer procedure, even when the weight of the apparatus""s payload is varied.
Another object of the present invention is to provide an apparatus for transferring spent nuclear fuel, the weight of which can be easily and quickly varied in order to maximize the amount of radiation shielding for a varied payload without substantially increasing the transfer procedure cycle time.
Yet another object of the present invention is to provide an apparatus for transferring spent nuclear fuel that can be lifted and transported by a low-capacity crane and still provide adequate radiation shielding during all stages of the transfer procedure.
Still another object of the present invention is provide a method of transferring a canister of spent nuclear fuel that provides the maximum amount of radiation shielding during all stages of the transfer procedure, even when the weight of the apparatus""s payload is varied.
Yet another object of the present invention is to provide a method of transferring a canister of spent nuclear fuel in nuclear power plants having a low capacity crane that provides adequate radiation shielding during all stages of the process.
These objects and others are met by the present invention which in one aspect is an apparatus for transferring spent, radioactive nuclear fuel comprising a cylindrical inner shell forming a cavity within which spent nuclear fuel can be placed; a cylindrical outer shell concentric with and surrounding the inner shell to form an annulus with the inner shell, the annulus adapted for receiving gamma absorbing material; a jacket shell concentric with and surrounding the second shell to form a jacket for holding a neutron absorbing liquid; and the jacket having a filling means and a drainage means.
It is preferable that the drainage means and filling means be adapted so that the jacket can be filled and drained with neutron absorbing liquid during a spent nuclear fuel transfer process. Preferably, the jacket shell of the apparatus has a top and a bottom, the filling means being located at or near the top of the jacket shell and the drainage means being located at or near the bottom of the jacket shell.
The filling means are preferably one or more holes capable of being hermetically sealed. Also preferably, the drainage means is one or more drain valves capable of being opened and hermetically sealed.
The gamma absorbing material is preferably lead and the neutron absorbing liquid is preferably water. The inner shell and outer shell are preferably constructed of carbon steel. It is also preferable that the apparatus further comprise a plurality of radial plates located within the jacket, wherein the radial plates connect the outer shell and the jacket shell. Preferably, the jacket shell is constructed of carbon steel. Also preferably, the annulus is filled with gamma absorbing material.
In another aspect, the invention is a method of transferring spent nuclear fuel from a pool comprising submersing an apparatus having a jacket, a cavity, and a canister within the cavity, for receiving spent nuclear fuel into a pool so that the canister fills with pool water, wherein the jacket is empty and hermetically sealed; placing spent nuclear fuel into the canister; lifting the transfer cask from the pool; placing the transfer cask in a staging area; filling the jacket with neutron absorbing liquid; draining the pool water from the canister, thereby reducing the weight of the transfer cask; and lifting the apparatus from the staging area.
Preferably, the method further comprises draining the neutron absorbing liquid from the jacket; hermetically sealing the jacket; and submersing the transfer cask in the pool for another load of spent nuclear fuel.
It is preferable that the jacket is drained by activating drainage means being located at or near the bottom of the jacket. Preferably, the drainage means is one or more drain valves which are hermetically sealed when the transfer cask is in the pool and after the jacket is filled.
Also preferably, the jacket is filled by introducing neutron absorbing liquid through the filling means being located near the top of the jacket. The filling means can be one or more holes which are hermetically sealed when the transfer cask is in the pool and after the jacket is filled.
The method preferably further comprises backfilling the canister with an inert gas once the pool water is drained. It is also preferable for the method to further comprise positioning the transfer cask above a storage cask; and transferring the canister from the transfer cask to the storage cask. Alternatively, the method can further comprise positioning the transfer cask above a transport cask; and transferring the canister from the transfer cask to the transport cask.